When to quit? well that is an individual thing-usually I quit when I mess up !! LOL Then I leave it for a day or so then go back to it -even if the fabric seems wrong at that moment or if it truly is wrong -that goes into a box for future use and then I pull out something else to match it hopefully if not then future usage of that material.
My eyes are the first to tell me when to quit .. LOL they say enough is enough, walk away .. LOL